In papermaking, paper or cardboard products are known to be formed by removing water from solid matter pulp. Measured in amounts, water is clearly the biggest raw-material, which is attempted to remove as quickly as possible from the end product (uncoated or coated paper or cardboard) in the wire, press and dryer sections. In papermaking, so-called high consistency pulp is typically first formed, mainly from fibres, water and inorganic fillers or pigments. Before the pulp is spread out in the head box and dewatering is started in the wire section, the high consistency pulp is diluted (typically to a consistency of 0.2-1.5%) to achieve better quality properties.
Dewatering is one of the most important factors that influence the economy of paper manufacture, and it is attempted to influence it chemically, among others using various flocculants and coagulants. Mechanical means of dewatering include, among others, suction boxes and drainage foils, which are intended to accelerate the dewatering process through the means of pulsating. Retention, which is closely related to dewatering, is used in defining the efficiency, by which, the solid matter can be removed from the papermaking process along with the paper or cardboard. An acceleration of the dewatering process and an increase in the solid matter retention improve the efficiency (of the drainage) of the paper machine. This should not, however, take place at the cost of deteriorating the quality of the cardboard. Formation is the measure of an even distribution of the solid matter. Formation and strength are some of the most important quality properties. A quicker dewatering in the wire section enables, among others, an increase in the velocity of the paper machine or the dilution of the head box and through this, the accomplishment of an improved formation. A more effective dewatering process also translates into a decrease in the need for drying energy in the dryer section.
In the paper or cardboard industry, for example colloidal-size calcium carbonate or calcium oxide or calcium hydroxide is used together with carbon dioxide to improve the properties of the end products, as known.
WO 2005/100690 A1 describes the use of calcium carbonate particles of an ultra fine (colloidal) size as a substitute for colloidal silicon dioxide with at least one natural or synthetic polymer to improve the dewatering of the paper pulp. The average particle size of this colloidal calcium carbonate is less than 200 nanometers.
EP 0344984 A2, describes the use of an aqueous colloidal calcium carbonate to improve the retention, drainage and formation in the manufacture of paper. The average particle size of this colloidal calcium carbonate is 100-300 nanometers. This reference discusses the colloidal calcium carbonate (PCC) that is made at a pH of 9-11 and is used together with cationic starch to improve the filler retention, drainage, and formation. In this manufacture of colloidal calcium carbonate, the anionic aspect is accomplished by an anionic dispersing agent (generally, an anionic, organic polymer), whereby, a hybrid product at an alkaline pH is formed, its surface chemistry essentially differing from the colloidal calcium carbonate in aqueous solution of the invention that contains at least bicarbonate.
US 2005257907 suggests that using calcium carbonate particles with an average particle size of less than 200 nm in finishing the paper surface, in connection with surface sizing or coating, results in a higher stiffness of the paper and less holes on the surface of the paper. The publication does not mention treating the process waters with carbonates in ionic state.
EP 0791685 A2 describes the precipitation of calcium carbonate on the surfaces of fibre and fines by adding carbon dioxide to a mixture of calcium hydroxide and paper furnish. As an end result, calcium carbonate crystals, of an average of 500 nanometers, precipitate on the surfaces of the fibre. When considering the results of table 3 of the publication, it can be observed that no improvement in the strength properties is achieved by the method of the publication. On the other hand, a particle of 0.5 micrometers corresponds to the normal particle size used in paper coating and is at least 3-5 times larger than the size category used in the present invention. The differences between the publication and the present invention include that the present invention does not aim to substitute the fibre with filler, but considerable economic advantages are still achieved.
FI 20085969 suggests that an improvement of dewatering, retention, and formation in the pH range of 6-9 is achieved in papermaking using the aqueous solution of colloidal calcium carbonate, bicarbonate, and other states of carbonate, when a charged polymer is used. According to the method of the publication, burnt lime or calcium hydroxide is first added to the process waters, whereafter the pH is lowered using carbon dioxide to the pH range of 6-9. This order of addition, which becomes evident from both the examples and the claims of the publication, and particularly the fact that the pH is not taken into consideration until after the other components have been added, causes variations in the solution pH during the manufacturing process. One weakness of the publication is that the pH variations are not taken into consideration in connection with the manufacturing stage of the composition, whereby problems with the runnability of the paper or cardboard machine, with precipitation, and variations in the brightness, are more likely. When using mechanical pulps, a weakening of the brightness in the alkaline pH range is also to be expected.
U.S. Pat. No. 7,056,419 describes the use of carbon dioxide in controlling the electrical properties of the paper manufacturing components, in order to decrease the amount of chemical additives used in the manufacture of paper. Carbon dioxide is preferably added to the refuse or calcium carbonate slurry. In the reference, the aim is generally to have a positive effect on the paper manufacturing conditions, so that the use of chemical additives could be decreased and, for example, the generation of unwanted reactions and the accumulation of chemicals in the white water system could be avoided. The method according to the publication is not, however, used in forming the colloidal calcium carbonate that is essential for achieving the advantages presented in the invention.